56 



TECHNICAL PROPERTIES UF WOOD. 



Fig. 6. 



Unevenly grained wood cracks more than wood which is even 

 grained, wood \N-ith broad summer-zones more than wood in which 

 the summer-zones are narrow. Sounding-wood for musical 

 instruments hardly cracks at all. 



As a rule, the cracks formed during shrinkage are pretty 

 straight : they arc rarely zigzag, as in old silver-fir wood, being 

 partly radial and partly tangential along the annual rings ; this 

 is also the case in old sprucewood from very high altitudes. 



Wood which is badly cracked owing to shrinkage may become 

 unsuitable for certain purposes. Cracks cannot be altogether 

 prevented, but by slow drying may become inconsiderable. This 

 may be effected by gradual barking, the bark being removed iu 

 strips, best of all in spirals, or the bark may be left for one 

 yard at either end of the log and in its 

 middle, and the rest barked. The best 

 method is to convert the wood into planks 

 and scantling while still green, and to 

 drv these pieces slowly. Wood thus 

 treated forms numerous small cracks, 

 but no large ones rendering it more or 

 less useless. 



In converting timber, the trunk should 

 be freed from sapwood and also from 

 its central part, which is specially liable 

 to crack (tig. 6), by cutting it iu halves 

 or quarters. These pieces when air- 

 dried may be converted into planks. The 

 conversion should follow the radial direction as much as pos- 

 sible, in order to prevent cracks. When wood is used for water- 

 pipes it must not be allowed to crack, and the best way to secure 

 this object is to bore it while green, and use it at once ; or if 

 intended for future use, to keep it under water till required. 

 Turners keep their freshly cut wood at first in cellars ; later, 

 in shady court-yards, and liually, in airy dry lofts. Experience 

 has often shown that beech-trees felled in spring and left lying 

 over summer with their crowns on, and thus drying gradually, 

 hardly crack at all. 



In order to prevent cracks at the ends of beams, planks and 

 other scantling, small pieces of wood may be naikd on to them. 



(After Boppc.) 



